Hazardous liquids, such as chemicals and petroleum products, are primarily transported over roadways in road tanker vehicles commonly referred to as cargo tanks. In particular, a petroleum cargo tank is typically divided into a plurality of separate cargo tank compartments for transporting one or more petroleum products. The most common type of petroleum cargo tank in use today is known as a "bottom loading" cargo tank because the cargo tank compartments are loaded and unloaded from the underside, or bottom, of the cargo tank. Each cargo tank compartment in a bottom loading cargo tank is equipped with an emergency valve located at the lowest point in the compartment that is biased in the closed position by a spring. The emergency valve is opened for loading and unloading the liquid petroleum product and is closed during transit to retain the product inside the cargo tank compartment.
An external conduit, referred to herein as a "wet line," is attached at one end to the emergency valve of the cargo tank by a fluid-tight connection such that the wet line is in flow communication with the interior of the cargo tank compartment. The wet line extends outwardly from the cargo tank compartment to a convenient location on the side of the cargo tank. The other end of the wet line terminates in a fluid-tight connection with the API adapter valve of a dry disconnect coupling that is biased in the closed position by a spring. At a loading facility, the liquid petroleum product is pumped under force through the dry disconnect coupling, the wet line and the emergency valve into the cargo tank compartment. After the cargo tank compartment is loaded and the API adapter valve and the emergency valve are closed, a residual amount of the petroleum product remains in the wet line. Depending on the distance between the API adapter valve and the emergency valve of a particular cargo tank compartment, the amount of residual liquid petroleum product can be several gallons. Since the residual liquid has already been metered and there is no convenient means for removing it from the wet line, the residual liquid remains in the wet line underneath the cargo tank during transit.
Wet lines, however, are susceptible to being damaged or broken off as a result of an accident. For example, there have been reported instances in which an automobile has impacted the underside of the cargo tank and caused one or more of the wet lines to become severed from the emergency valve. For this reason, the wet line is designed to fail in an underside collision at a point outside the cargo tank compartment adjacent the emergency valve, thereby preventing the entire contents of the compartment from being emptied. When the emergency valve or wet line fails, however, the residual liquid in the wet line is spilled onto the ground underneath the cargo tank. As a result, there is significant potential for contamination of the ground, including the underground water supply, by the liquid petroleum product.
One solution to this problem is proposed in U.S. Pat. Nos. 5,462,078 and 5,377,715 to Andenmatten et al. The Andenmatten et al. patents disclose an apparatus and method for eliminating hazardous materials from cargo tank wet lines. In particular, the Andenmatten et al. patents disclose an apparatus and method for purging hazardous materials from the wet lines of a petroleum cargo tank using a pressurized gas. The pressurized gas forces the residual liquid to the lowest point in the wet line where it is directed through a purging conduit back into the cargo tank compartment. The pressurized gas may be obtained from an external source, such as compressed air from the tractor or the loading facility, or may be created by pressurizing petroleum vapors collected by the cargo tank's vapor recovery system during the loading process. The Andenmatten et al. patents further disclose an initiation and shut off system for automatically purging the residual liquid petroleum product from the wet line once the cargo tank compartment is loaded and the interference gate on the side of the cargo tank is closed.
Government legislation is anticipated that will likely require all bottom loading cargo tanks within the United States to be operated in a manner that eliminates the danger of ground contamination by hazardous liquids during transit. With the present configuration of bottom loading cargo tanks, such potential legislation would require each wet line carrying hazardous liquid to be purged of residual liquid when the cargo tank departs the loading facility and while the cargo tank is in transit. Accordingly, in the event that the wet line is damaged or broken off as a result of an accident, very little, if any, hazardous liquid will be spilled onto the ground in the vicinity of the cargo tank. Therefore, the danger of ground contamination would be eliminated.
The apparatus and method disclosed in the Andenmatten et al. patents merely ensure that the wet lines are purged of residual liquid immediately following the loading process at the loading facility. Unfortunately, a great number of the emergency valves utilized to seal the cargo tank compartments of bottom loading cargo tanks are highly susceptible to leakage during transit. As a result, it is likely that one or more of the wet lines on the cargo tank will become wet during transit as the hazardous liquid leaks from the emergency valve and collects in the wet line. As such, it is still possible that a significant amount of hazardous liquid will spill in the event that one or more wet lines are damaged or broken off as a result of an accident. Accordingly, the danger of ground contamination is not entirely eliminated by the method and apparatus proposed by the Andenmatten et al. patents.
It is thus apparent that a system and method is needed for entirely eliminating the danger that hazardous liquid will be spilled during transit in the event that a wet line is damaged or broken off More particularly, it is apparent that a system and method is needed for ensuring that the wet lines of a bottom loading petroleum cargo tank remain free from hazardous liquid during transit. It is further apparent that a system and method is needed for continuously monitoring the presence of residual liquid in a wet line of a bottom loading cargo tank and for automatically purging the residual liquid from the wet line during transit. It is further apparent that a system and method is needed for indicating to the operator of a bottom loading cargo tank when the emergency valve of a cargo tank compartment is leaking hazardous liquid into a wet line.